Drop luggage and orient yourself at Piazzale Roma; this transport hub is an ideal starting point for walking or catching vaporettos. Gives quick access to the entire city.
Stroll across the iconic bridge and browse the morning fish/produce stalls to experience local color and grab quick snacks; markets generally open by 7:30–8:00am and wind down late afternoon.
Visit Venice's principal square and the Basilica di San Marco (check basilica hours; usually 9:30am–5:00pm non-worship entry); the mosaics and architecture are must-sees.
Explore the exterior and small exhibitions around the historic shipyards; the Arsenale area is largely open public space, with some events/exhibits seasonally open (check schedules).
If the Biennale is on, visit its pavilions (seasonal); otherwise the Naval History Museum offers naval artifacts (check opening hours, usually ~9:30–17:30).
Visit the ornate Scuola and lively Campo Santa Maria Formosa to see local life and Renaissance architecture; sites typically open afternoon hours ~14:00–17:00.